The present invention is directed towards a papermaker""s fabric or wire, particularly one for use in a dual nip stock thickening device.
In the papermaking industry, there is an apparatus which is used to thicken pulp and paper stock. Early on such devices were commonly referred to as deckers. These early devices involved the use of cylinder molds which included a porous cylinder mold rotating in a vat of liquid with a controlled input of slurry. Water would be drained off through the cylinder mold thus thickening the remaining slurry which would be drained off. An example of this type of device can be found in U.S. Pat. No. 4,106,980.
An improvement on the then conventional thickeners can be found in U.S. Pat. No. 4,722,793. This patent describes a device which avoids the use of a cylinder mold. It employs a single pair of smooth-surfaced rolls and a single fabric trained around those rolls so that it wraps substantially 180xc2x0 of the surface of each roll. The pulp stock to be thickened is initially delivered to the inside of a fabric run approaching the top of one roll so that the pulp is trapped in a zone between the fabric and the roll and is made to travel around the roll with the fabric. Centrifugal force causes liquid to be expressed through the fabric from the pulp trapped between the fabric and the roll.
The resulting partially dewatered pulp then travels on a lower fabric run to the other roll, where it is similarly subjected to centrifugal force causing further expression of liquid through the fabric. After travelling around the surfaces of both rolls, the pulp is removed from the surface of the second roll.
In order to guide the fabric in a path perpendicular to the axes of the two rolls, the fabric was provided along one or both of its edges on its bottom surfaces with a strip of material or guide in the shape of a V-belt. This guide was intended to fit into a peripheral groove in each of the rolls.
This belt is made separate from the fabric and mechanically attached thereto. Due, however, to the high- speed operation of the device, difficulty was encountered in maintaining the guide on the fabric.
In an effort to improve on this arrangement, U.S. Pat. No. 5,039,412 teaches providing for stitching the V-belt guide to the fabric and providing a band of adhesive on the fabric in the area of the stitching. The application of the adhesive extends a short distance inward from the side of the fabric. Also, guides are provided on the outer edge of the fabric so that they are positioned on the outside of the rolls rather than in a groove or grooves in the rolls.
While this design may have improved upon earlier designs by reinforcing the edges and supporting the V-belt guides, it has been found that the fabric tends to fail where the adhesive ends. It appears that the edges (with the guides) bounce during operation. This results in flexing of the fabric which is concentrated at the transition of the edging to the body of the fabric. Such a failure typically results in the edging (with the guide) separating from the body of the fabric.
It is therefore a principle object of the invention to provide for a papermaker""s fabric which is reinforced at the edges thereof in a manner that improves the fabrics life.
It is a further object of the invention to provide for such reinforcement in a manner that is readily implemented and cost effective.
A further object of the invention is to provide for such reinforcement on a fabric used on a papermaker""s pulp thickening apparatus.
The present invention provides for such objects and advantages in the following manner. In situations where the papermaker""s fabric is subject to stress on its edges, a reinforcing or protective coating is provided on one or both sides of the fabric. More particularly, oftentimes the stress is located at a predominant location on the fabric. For example, with the aforesaid fabric for a thickening device, stress would be located at the transition between the edge or edge portions and the body of the fabric and would typically run the length of the fabric parallel to the edge. The present invention envisions a reinforcing coating to extend beyond the transition of the edge and body. This additional amount of coating would vary in the extent to which it extends into the body. Also, it terminates in the form of a series of curves, circle segments, or other projections. This allows for the stress (or flexing at the transition) to be distributed over a greater area which reduces or minimizes the hinging or pivoting effect of the edge against the body of the fabric.